Test gas is defined as an exactly mixed, precision gas mixture with a defined composition, i.e., a composition that is known in terms of the type and quantity of its components. In many cases it consists of a basic gas or basic gas mixture that contains the main component of the test gas and one or more admixtures, which are used directly for testing or calibration.
Test gases are needed with different admixtures, whose types, numbers, and concentrations vary widely. The range of applications for test gases extends from the calibration and adjustment of measurement devices, to process optimization and system monitoring, to research and development, to medicine.
Test gases are mainly produced by combining defined quantities of different gas fractions. The oldest production methods are manometric and volumetric in nature. The manometric process is based on the pressure change that occurs after the individual admixtures or the basic gas are added. Conversion with the aid of phenomenological gas state equations is required in order to precisely determine the mass concentrations. In the case of the volumetric process, the volumes of the individual gas mixture components are determined using, e.g., flowmeters and are transferred to a gas container, hereinafter referred to as a flask. With both processes, however, the precision of mixing is relatively low, so that additional gas analysis has to be carried out in order to precisely determine the gas composition.
With the availability of sufficiently accurate scales, the gravimetric production of test gases quickly became common and is now generally preferred. The principle is that the gas mixture components are put into the flask one after the other, and after each addition the increase in mass is determined by weighing. This provides the direct reference between the gases that are weighed in and the base values "kg" and "mol", and then it is no longer necessary to convert using state equations. Since mass determinations by means of weighing are among the most accurate of physical measuring processes, test gases of extremely high precision can be produced with this method.
In practice, the gravimetric production of test gases is done in such a way that an evacuated flask is placed on a scale, and the connection is made to the gas supply, gas metering, and control system by means of a gas feed line, generally a capillary with which any influence on the weighing process is kept to an absolute minimum. The subsequent procedure is distinguished depending on whether reweighing is done or not.
When production is done without reweighing, the gas feed line required for supplying gas is first connected to the flask, and a first weighing is carried out. Then the first component according to the calculated recipe of the test gas that is to be produced is put in. Usually, the components are put in in the sequence of rising concentration. Once the filling procedure has been completed, the flask with the attached gas feed line and the first component that it contains are weighed again. The difference between the two weighings of the flask before and after it is filled yields the mass of the first component that it contains. These steps are then suitably repeated for all subsequent test gas components.
When the weighings are done with the gas feed line attached, however, the test results are adulterated owing to the influence of the tension in the gas feed line on the scale. In general, therefore, the values that are determined in this way cannot be used to certify the composition. Rather, such certification is established by subsequent physical-chemical analysis against a test gas standard.
In order to correct this deficiency, use is made of the process of gravimetric test gas production with reweighing. In this case, before and after each component is put in, the gas feed line is detached from the flask, and a so-called reweighing is carried out. Since in each case the scale is completely detached from the gas supply, only the flask with the corresponding contents and without perturbing external influences is weighed. The values that are determined from reweighing can thus be used to determine exactly the masses of the individual components. With this process, the certification of composition is accomplished strictly by means of determination of mass. Final gas-analysis tests are therefore no longer needed.
Up until now, gravimetric test-gas production with reweighing has been done manually and is therefore very labor-intensive and time-intensive. It has not yet been possible to automate the process.
From DE-C-37 39 950 a process is known for automatically decanting gases into flasks after their weights are taken. To do this, a rotary head for opening and closing the flask valve and a fill head for putting in the gas are set up. When the flask is filled with gas, gas is put into the flask in several steps and then weighed until the specified end weight is reached. The above-described decanting method is not, however, suitable for the production of precision gas mixtures since a gas hose, a compressed-air hose, and a power cable extend out from the scale and affect the weighing process.